I'm guessing the only advantage is Ditto being gifted in intelligence
Intelligence: Instinctual Savant (Mahoraga operates on a binary of feral aggression and calculated adaptation. In its untamed state, it attacks with the indiscriminate ferocity of a wild beast, lacking any discernable higher thought beyond destruction. Yet, this savagery masks a sophisticated analytical engine capable of decoding the fundamental logic of any phenomena it faces. Under the control of a sorcerer, this duality shifts: Mahoraga transitions from a mindless berserker to a highly intuitive guardian, displaying enough situational awareness to intercept threats and cover its user's blind spots apparently on its own initiative)
Weird intelligence justification.
"Yet, this savagery masks a sophisticated analytical engine capable of decoding the fundamental logic of any phenomena it faces."
Is it capable of "decoding the fundamental logic of any phenomena it faces" in the feral aggression half of the binary of its intelligence? If not, it saying said feral aggression masks it only applies if that intellectual capacity is only able to be applied under the control of a sorcerer.
Ditto isn't much better:
Intelligence: Below Average normally (Capable of coherent speech between other Pokémon, a degree of understanding of commands and social situations, and can solve minor puzzles, though still mostly driven by instinct),
Above Average in combat (Pokémon are hard-wired for battle, even in the wild can master many moves and replicate the likes of
Machop which train in all martial arts)
In theory, it might be skilled & able to use Mahoraga's powers well, since Ditto can effectively use moves of Pokemon they turn into, but it probably won't know which to use, as it won't know Mahoraga is invulnerable to anything but Cursed Energy.
Stamina: Superhuman (Mahoraga can only be killed
by being completely destroyed in one hit[16], otherwise he will survive and adapt.
Survived from Sukuna's Dismantles that slashed into it's head[1] and was later able to survive Malevolent Shrine despite Cleave
visibly damaging him[16])
Stamina: Superhuman (The average Pokemon is hardwired for battle and can go on fighting for a long time despite being injured. Capable of spending a day on foot while fighting hordes of enemies in mystery dungeons with little to no breaks. Can swim between islands and continents with no breaks)
So we have an opponent Ditto can't see from the start, doesn't know it's invulnerable, doesn't know it needs to one-shot it, that adapts to other stuff.
The only advantages it has are a small Intelligence/Skill advantage, maybe & a Stamina advantage, I think.